56 Where did Life Begin f 



have swept from the poles to the equator — one 

 after the other, and age after age, through a 

 period so vast, so incomprehensible, that even 

 the epochs of which it is composed seem to 

 us almost like so many eternities. Let us 

 draw near, observe and define more closely 

 this wonderful force, an excess of which for- 

 bids life, a deficiency of which destroys life, 

 and without which life is impossible. 



Heat is the proximate cause of all activity. 

 With it life comes and goes. The departure 

 of heat which we call '' cold " is death. Cold 

 huddles and quiets the molecules of all known 

 substances. Life cannot invade its precincts. 

 As cold dispersed the plants and animals 

 from the polar regions, so it has set up an 

 impassable barrier against their return. Every 

 high mountain has a cold line above which 

 nothing thrives. The earth is growing colder 

 age by age. Our next neighbor, the moon, 

 has already become cold and lifeless, while 

 yet receiving its full proportion of sun-heat. 

 Certain e^'ades or deo^rees of heat then not 

 only constitute the moving power of all life, 

 but does it not seem probable that the con- 



